Transcript Chapter 2 Primitive Types, Strongs, and Console I/O

Walter Savitch
Frank M. Carrano
Basic Computation
Chapter 2
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Objectives …
• Describe the Java data types used for simple data
• Write Java statements to declare variables, and define
named constants
• Write assignment statements, and expressions containing
variables and constants
• Understand “assignment compatibility” and learn how to
“cast” values from one type to another
• Understand arithmetic operations, use of parentheses, rules
of precedence, and the special increment and decrement
operators
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Objectives
• Understand use of the specialized assignment operators
• Define strings of characters, and perform simple string
processing
• Write Java statements that accomplish keyboard input and
screen output
• Adhere to stylistic rules, guidelines and conventions
• Write meaningful comments
• Case Study: Vending Machine Change
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Data Types
• A primitive type is used for simple,
nondecomposable values such as an individual
number or individual character.
 int, double, and char are primitive types.
• A class type is used for a class of objects and
has both data and methods.
 "Java is fun" is a value of class type String
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Primitive Types and Values
• Four integer types:
byte, short, int, and long
 int is most common and the default
 (0
-1
365
12000)
• Two floating-point types: float and double
 double is more common and the default
 (0.99
-22.8
3.14159 5.0)
• One character type: char ('a' 'A' '#' ' ')
• One boolean type: boolean (true false)
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
All Primitive Types
• Figure 2.1 Primitive Types
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
e Notation
• e notation is also called scientific notation or
floating-point notation.
• Examples
 865000000.0 can be written as 8.65e8
 0.000483 can be written as 4.83e-4
• The number in front of the e does not need to
contain a decimal point.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Imprecision in Floating-Point Numbers
• Floating-point numbers often are only
approximations since they are stored with a finite
number of bits.
• Hence 1.0/3.0 is slightly less than 1/3.
• 1.0/3.0 + 1.0/3.0 + 1.0/3.0
is less than 1.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Java Identifiers
• An identifier is a name, such as the name of a
variable.
• Identifiers may contain only
 Letters
 Digits (0 through 9)
 The underscore character (_)
 And the dollar sign symbol ($) which has a special
meaning
• The first character cannot be a digit.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Java Identifiers
• Identifiers may not contain any spaces, dots (.),
asterisks (*), or other characters:
7-11 netscape.com util.* (not allowed)
• Identifiers can be arbitrarily long.
• Since Java is case sensitive, stuff, Stuff,
and STUFF are different identifiers.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Keywords or Reserved Words
• Words such as if are called keywords or
reserved words and have special, predefined
meanings.
 Cannot be used as identifiers.
 See Appendix 1 for a complete list of Java keywords.
• Example keywords: int, public, class
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Naming Conventions
• Class types begin with an uppercase letter
(e.g. String).
• Primitive types begin with a lowercase letter
(e.g. int).
• Variables of both class and primitive types begin
with a lowercase letters
(e.g. myName, myBalance).
• Multiword names are "punctuated" using
uppercase letters.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Variables
• Variables store data such as numbers and
letters.
 Think of them as places to store data.
 They are implemented as memory locations.
• The data stored by a variable is called its value.
 The value is stored in the memory location.
• The value of a variable can be changed by
assignment (or in other ways).
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Naming and Declaring Variables
• Choose names that are helpful such as count
or speed, but not c or s.
• When you declare a variable, you provide its
name and type.
int numberOfBaskets,eggsPerBasket;
• A variable's type determines what kinds of
values it can hold (int, double, char, etc.).
• A variable must be declared before it is used.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Syntax and Examples
• Syntax
type variable_1, variable_2, …;
• Examples
int styleChoice, numberOfChecks;
double balance, interestRate;
char jointOrIndividual;
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Where to Declare Variables
• Declare a variable
 Just before it is used or
 At the beginning of the section of your program that is
enclosed in {}.
public static void main(String[] args)
{ /* declare variables here */
. . .
}
• Don’t confuse: declaring a variable, initializing a
variable, and assigning to a variable
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Initializing Variables
• A variable that has been declared, but not yet
given a value is said to be uninitialized.
• Java won’t let you use an uninitialized variable.
• Uninitialized class variables have the value
null.
• Uninitialized primitive variables may have a
default value but it's good practice not to rely on
a default value.
• Examples (but one per line is better):
int count = 0, size = 7;
char grade = 'A';
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Assignment Statements
• Syntax (expressions on the right are evaluated before
assignment takes place)
variable = expression
answer = 42;
firstInitial = 'W';
score = numberOfCards + handicap;
where expression can be another variable, a literal or
constant (such as a number), or something more
complicated which combines variables and literals using
operators
(such as + and -)
• The "equal sign" is called the assignment operator.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Variable Sample Program
• View sample program listing 2.1
 Class EggBasket
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Variable Sample Program with
Simple Input
• View sample program listing 2.2
class EggBasket2
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Constants
• Literal expressions such as 2, 3.7, or 'y' are
called constants.
• Integer constants can be preceded by a + or sign, but cannot contain commas.
• Floating-point constants can be written
 With digits after a decimal point or
 Using e notation.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Named Constants
• Java provides mechanisms to …
 Declare a variable
 Initialize a variable
 Assign to a variable
 Fix the value of a variable so it cannot be
changed
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Using Named Constants
• To avoid confusion, always name constants as
well as variables:
area = PI * radius * radius;
is clearer than
area = 3.14159 * radius * radius;
• Place constants near the beginning of the
program.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Why name a constant?
• Once the value of a constant is set (or changed
by an editor), it can be used (or reflected)
throughout the program.
public static final double INTEREST_RATE = 6.65;
• If a literal (such as 6.65) is used instead, every
occurrence must be changed, with the risk than
another literal with the same value might be
changed unintentionally.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Declaring Named Constants
• Syntax
public static final Data_Type
NAMED_CONSTANT = literal_value;
• Examples
public static final double
PI = 3.14159;
public static final String MOTTO =
"The customer is always right.";
• By convention, uppercase letters are used for
constants.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Named Constants
Sample Program
• View sample program
class CircleCalculation2, listing 2.8
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Assignment Compatibilities …
• Java is said to be strongly typed.
 You can't, for example, assign a floating point value to
a variable declared to store an integer.
• Sometimes conversions between numbers are
possible.
doubleVariable = 7;
is possible even if doubleVariable is of type
double, for example.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Assignment Compatibilities
• A value of one type can be assigned to a
variable of any type further to the right
byte --> short --> int --> long
--> float --> double
 But not to a variable of any type further to the left.
• You can assign a value of type char to a variable
of type int.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Type Casting …
• A type cast temporarily changes the value of a
variable from the declared type to some other
type.
• For example,
double distance;
distance = 9.0;
int points;
points = (int)distance;
• Illegal without (int)
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Type Casting
• The value of (int)distance is 9,
• The value of distance, both before and after
the cast, is 9.0.
• Any nonzero value to the right of the decimal
point is truncated rather than rounded.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Arithmetic Operators
• Arithmetic expressions can be formed using the
+, -, *, and / operators together with
variables or numbers referred to as operands.
 When both operands are of the same type, the result
is of that type.
 When one of the operands is a floating-point type and
the other is an integer, the result is a floating point
type.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Arithmetic Operations …
• Example
If hoursWorked is an int to which the value 40
has been assigned, and payRate is a double to
which 8.25 has been assigned, then
hoursWorked * payRate
is a double with a value of 500.0.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Arithmetic Operations …
• Expressions with two or more operators can be
viewed as a series of steps, each involving only
two operands.
 The result of one step produces one of the operands
to be used in the next step.
• Example
balance + (balance * rate)
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Arithmetic Operations
• If at least one of the operands is a floating-point
type and the rest are integers, the result will be a
floating point type.
• The result is the rightmost type from the
following list that occurs in the expression.
byte --> short --> int --> long
--> float --> double
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Division Operator
• The division operator (/) behaves as expected if
one of the operands is a floating-point type.
• When both operands are integer types, the
result is truncated, not rounded.
 Hence, 99/100 has a value of 0.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The mod Operator
• The mod (%) operator is used with operators of
integer type to obtain the remainder after integer
division.
• 14 divided by 4 is 3 with a remainder of 2.
 Hence, 14 % 4 is equal to 2.
• The mod operator has many uses, including
 determining if an integer is odd or even
 determining if one integer is evenly divisible by
another integer.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Parentheses and Precedence
• Parentheses can communicate the order in
which arithmetic operations are performed
• Examples:
(cost + tax) * discount
cost + (tax * discount)
• Without parentheses, an expressions is
evaluated according to the rules of precedence.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Sample Expressions
• Figure 2.3 Some Arithmetic Expressions in Java
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Specialized Assignment
Operators
• Assignment operators can be combined with
arithmetic operators (+, -, *, /, and %).
amount = amount + 5;
can be written as
amount += 5;
yielding the same results.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Increment and Decrement
Operators …
• Used to increase (or decrease) the value of a
variable by 1
• Easy to use, important to recognize
• The increment operator
count++ or ++count
• The decrement operator
count-- or --count
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Increment and Decrement
Operators
• Equivalent operations
count++;
++count;
count = count + 1;
count--;
--count;
count = count - 1;
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Increment and Decrement
Operators in Expressions
• after executing
int m = 4;
int result = 3 * (++m)
result has a value of 15 and m has a value of 5
• after executing
int m = 4;
int result = 3 * (m++)
result has a value of 12 and m has a value of 5
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Precedence Rules
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Precedence Rules
• When unary operators have equal precedence,
the operator on the right acts before the
operation(s) on the left.
• Even when parentheses are not needed, they
can be used to make the code clearer.
balance + (interestRate * balance)
• Spaces also make code clearer
balance + interestRate*balance
but spaces do not dictate precedence.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Precedence Rules
• The binary arithmetic operators *, /, and %,
have lower precedence than the unary operators
+, -, ++, --, and !, but have higher
precedence than the binary arithmetic operators
+ and -.
• When binary operators have equal precedence,
the operator on the left acts before the
operator(s) on the right.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Case Study:
Vending Machine Change …
• Requirements
 The user enters an amount between 1 cent and 99
cents.
 The program determines a combination of coins equal
to that amount.
 For example, 55 cents can be two quarters and one
nickel.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
• Sample dialog
Enter a whole number from 1 to 99.
The machine will determine a
combination of coins.
87
87 cents in coins:
3 quarters
1 dime
0 nickels
2 pennies
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
• Variables needed
int amount,
quarters,
dimes,
nickels,
pennies;
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
•
Algorithm - first version
1. Read the amount.
2. Find the maximum number of quarters in
the amount.
3. Subtract the value of the quarters
from the amount.
4. Repeat the last two steps for dimes,
nickels, and pennies.
5. Print the original amount and the
quantities of each coin.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
• The algorithm doesn't work properly
 Original amount is changed by the intermediate steps.
 Original value of
amount is lost.
• Change the list of variables
int amount, originalAmount,
quarters, dimes, nickles, pennies;
 Update the algorithm.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
•
Algorithm – second version
1. Read the amount.
2. Make a copy of the amount.
3. Find the maximum number of quarters in
the amount.
4. Subtract the value of the quarters
from the amount.
5. Repeat the last two steps for dimes,
nickels, and pennies.
6. Print the original amount and the
quantities of each coin.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
• View Java code that implements the algorithm
written in pseudocode – listing 2.3
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
• How do we determine the number of quarters (or
dimes, nickels, or pennies) in an amount?
• There are 2 quarters in 55 cents, but there are
also 2 quarters in 65 cents.
• That's because
55 / 25 = 2 and 65 / 25 = 2.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study …
• How do we determine the remaining amount?
• The remaining amount can be determined using
the mod operator
55 % 25 = 5 and 65 % 25 = 15
• Similarly for dimes and nickels.
• Pennies are simply amount % 5.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Case Study
• The program should be tested with several
different amounts.
• Test with values that give zero values for each
possible coin denomination.
• Test with amounts close to
 extreme values such as 0, 1, 98 and 99
 coin denominations, such as 24, 25, and 26.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Comments Example
• View sample program
class CircleCalculation, listing 2.7
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Class String
• We've used constants of type String already.
"Enter a whole number from 1 to 99."
• A value of type String is a
 Sequence of characters
 Treated as a single item.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
String Constants and Variables
• Declaring
String greeting;
greeting = "Hello!";
or
String greeting = "Hello!";
or
String greeting = new
String("Hello!");
• Printing
System.out.println(greeting);
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Concatenation of Strings
• Two strings are concatenated using the +
operator.
String greeting = "Hello";
String sentence;
sentence = greeting + " officer";
System.out.println(sentence);
• Any number of strings can be concatenated
using the + operator.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Concatenating Strings and
Integers
String solution;
solution = "The answer is " + 42;
System.out.println (solution);
The temperature is 72
The answer is 42
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
String Methods
• An object of the String class stores data
consisting of a sequence of characters.
• Objects have methods as well as data
• The length() method returns the number of
characters in a particular String object.
String greeting = "Hello";
int n = greeting.length();
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Method length()
• The method length() returns an int.
• You can use a call to method length() anywhere
an int can be used.
int count = command.length();
System.out.println("Length is " +
command.length());
count = command.length() + 3;
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
String Indices
• Figure 2.4
• Positions start with 0, not 1.
• The 'J' in "Java is fun." is in position 0.
• A position is referred to an an index.
• The 'f' in "Java is fun." is at index 8.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
String Methods …
Figure 2.5a
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… String Methods …
Figure 2.5b
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… String Methods …
Figure 2.5c
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… String Methods
Figure 2.5d
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
String Processing
• No methods allow you to change the value
of a String object.
• But you can change the value of a
String variable.
• View sample program StringDemo
listing 2.4
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Escape Characters …
• How would you print
"Java" refers to a language.
?
• The compiler needs to be told that the quotation
marks (") do not signal the start or end of a
string, but instead are to be printed.
System.out.println(
"\"Java\" refers to a language.");
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Escape Characters
• Figure 2.6
• Each escape sequence is a single character
even though it is written with two symbols.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Examples
System.out.println("abc\\def");
abc\def
System.out.println("new\nline");
new
line
char singleQuote = '\'';
System.out.println
(singleQuote);
'
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Unicode Character Set
• Most programming languages use the ASCII
character set.
• Java uses the Unicode character set which
includes the ASCII character set.
• The Unicode character set includes characters
from many different alphabets (but you probably
won't use them).
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Using the Scanner Class
• Near the beginning of your program, insert
import java.util.Scanner;
• Create an object of the Scanner class
Scanner keyboard =
new Scanner (System.in)
• Read data (an int or a double, for example)
int n1 = keyboard.nextInt();
double d1 = keyboard.nextDouble();
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Keyboard Input Demonstration
• View sample program
class ScannerDemo, listing 2.5
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Some Scanner Class Methods…
• Figure 2.7a
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Some Scanner Class Methods
• Figure 2.7b
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
nextLine()Method Caution …
• The nextLine() method reads
 The remainder of the current line,
 Even if it is empty.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…nextLine()Method Caution
• Example – given following declaration.
int n;
String s1, s2;
n = keyboard.nextInt();
s1 = keyboard.nextLine();
s2 = keyboard.nextLine();
• Assume input shown
42
and don't you
forget it.
n is set to 42
but s1 is set to the empty string.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Empty String
• A string can have any number of characters,
including zero.
• The string with zero characters is called the
empty string.
• The empty string is useful and can be created in
many ways including
String s3 = "";
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Other Input Delimiters …
• Almost any combination of characters and
strings can be used to separate keyboard input.
• to change the delimiter to "##"
keyboard2.useDelimiter("##");
 whitespace will no longer be a delimiter for
keyboard2 input
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Other Input Delimiters
• View sample program
class DelimitersDemo, listing 2.6
Sample
Screen
Output
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Summary …
• You have become familiar with Java
primitive types (numbers, characters, etc.).
• You have learned about assignment
statements and expressions.
• You have learned about strings.
• You have become familiar with classes,
methods, and objects.
• You have learned about simple keyboard
input and screen output.
JAVA: An Introduction to Problem Solving & Programming, 5 th Ed. By Walter Savitch and Frank M. Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved